AlMe3 and ZnMe2 Adducts of a Titanium Imido Methyl Cation:  A Combined Crystallographic, Spectroscopic, and DFT Study

A combined experimental and DFT study of the reactions of the titanium imido methyl cation [Ti(NtBu)(Me3[9]aneN3)Me]+ (4+) with AlMe3 and ZnMe2 is described. Reaction of 4+ with AlMe3 gave [Ti(NtBu)(Me3[9]aneN3)(μ-Me)2AlMe2]+ (7+), the first structurally characterized AlMe3 adduct of a transition metal alkyl cation and a model for the presumed resting state in MAO-activated olefin polymerizations. Reaction of 4+ with ZnMe2 also gave a methyl-bridged heterobinuclear species, namely [Ti(μ-NtBu)(Me3[9]aneN3)(μ-Me)2ZnMe]+ (8+), the first directly observed ZnMe2 adduct of a transition metal alkyl cation. At room temperature, all three metal-bound methyls of 8+ underwent rapid exchange with those of free ZnMe2, whereas at 233 K only the terminal Zn−Me group exchanged significantly. Addition of AlMe3 to 8+ quantitatively formed 7+ and ZnMe2. Reaction of 4+ with Cp2ZrMe2 gave [Ti(NtBu){Me2(μ-CH2)[9]aneN3}(μ-CH2)ZrCp2]+ (10+) via a highly selective double C−H bond activation reaction in which both alkyl groups of Cp2ZrMe2 were lost. DFT calculations on models of 7+ confirmed the approximately square-based pyramidal geometries for the bridging methyl groups. Calculations on 8+ found that the formation of the Ti(μ-Me)2Zn moiety is assisted by an Nimide→Zn dative bond. DFT calculations for the sterically less encumbered methyl cation [Ti(NMe)(H3[9]aneN3)Me]+ found strong thermodynamic preferences for adducts featuring Nimide→M (M = Al or Zn) interactions. This offers insight into recently observed structure−productivity trends in MAO-activated imido-based polymerization catalysts. Calculations on the metallocenium adducts [Cp2Ti(μ-Me)2AlMe2]+ and [Cp2Ti(μ-Me)2ZnMe]+ are described, each showing α-agostic interactions for the bridging methyl groups. For these systems and the imido ones, the coordination of AlMe3 to the corresponding monomethyl cation is ca. 30 kJ mol-1 more favorable than for ZnMe2.